Question

Arrange the following respiratory substrates in increasing order based on ATPs they give on complete oxidation in three stages of aerobic respiration:
I: G-3-P (Glyceraldehyde 3-Phosphate)
II: Acetyl CoA
III: Acetyl CoA
IV: \(\alpha\)-Ketoglutarate (\(\alpha\)KGA)

• A. II, IV, I, III
• B. III, II, I, IV
• C. I, III, II, IV
• D. IV, III, II, I

Hint:

Remember, the location and role of a substrate in metabolic pathways heavily influence its ATP yield. Substrates involved in later stages like the Krebs cycle generally contribute more to ATP production.

Answer 1

The Correct Option is D

Step 1: The complete oxidation of respiratory substrates involves three stages: Glycolysis, the Citric Acid Cycle, and Oxidative Phosphorylation (Electron Transport Chain). Each substrate generates ATP based on the number of molecules processed through these stages. 
Step 2: G-3-P (Glyceraldehyde 3-Phosphate) enters glycolysis and produces fewer ATP molecules. After glycolysis, it is converted into pyruvate and then acetyl CoA, which enters the Citric Acid Cycle. Hence, G-3-P yields the least ATP. 
Step 3: Acetyl CoA can enter the Citric Acid Cycle and undergo complete oxidation to generate ATP. However, compared to other substrates, it gives a moderate amount of ATP. Acetyl CoA is involved in the Citric Acid Cycle, where it is further oxidized to produce NADH, FADH2, and ATP. 
Step 4: \(\alpha\)-Ketoglutarate (\(\alpha\)KGA) is a key intermediate in the Citric Acid Cycle and contributes significantly to ATP generation. It is converted into succinyl-CoA, which then undergoes further steps to produce NADH and ATP. 
Conclusion: Based on the ATP yield on complete oxidation: - IV. \(\alpha\)-Ketoglutarate (\(\alpha\)KGA) produces the most ATP, - III. Acetyl CoA comes next, - II. Acetyl CoA and I. G-3-P yield ATP in increasing order. Hence, the correct order is IV, III, II, I.